Method for determining parameters for the design of ground equipment for a satellite link system and a method of transmitting signals in a satellite link system

ABSTRACT

A method of determining parameters for the design of ground equipment for satellite link systems using duplex links is disclosed. The method has particular application to determining transponder power requirements and antenna size. The method comprises selecting required availability of the satellite system and ground sites for the equipment and determining site degradation parameters for the ground sites. A range where no up link power control is required is also determined and a point of minimum transponder power is determined to enable the parameters to be selected.

BACKGROUND OF THE INVENTION

This invention relates to a method of determining parameters for thedesign of ground equipment for satellite link systems utilising duplexlinks.

DESCRIPTION OF THE PRIOR ART

Typical satellite link systems for duplex links include at least twoground stations and an orbiting satellite. The ground stations transmitand receive signals to and from one another via the satellite. Typicalsatellite link designs for duplex links implement a duplex link as twoseparate simplex links. When signals are transmitted in the higherbands, for example, the Ku or Ka band (11 to 14 GHz) the signals can beaffected by rain and therefore this must be taken into account whendesigning satellite link systems. A location in which this factor mustbe considered in designing satellite link systems is in tropical regionswhere heavy rain can be expected in some parts of the year. Theinvention also has application in semi and non tropical regions as well.

In order to ensure that the system will be available for use in suchenvironments systems are normally designed to operate or to be availablea specific proportion of time, say 99.5% of the time. Availability 99.5%of the time is acceptable to commercial users utilising the satellitelink system for communication purposes such as telephone communicationand other types of communication. Thus, the need to ensure that thesystem is available 99.5% of the time means that the ability to transmitwhen rain is present at one or both of the ground stations must be takeninto account when designing the ground equipment and in particulartransmitter power requirements and antenna size. Minimization oftransmitter power requirements and also antenna size is most desirableand represents significant cost saving in both production of the groundsite and also use of the equipment at the ground site during operationof the system.

Conventional systems employ a facility known as uplink power control toincrease availability by reducing the effect of rain fading on uplinksignals transmitted from the ground station to the satellite. Uplinkpower control basically alters the output power of the transmitter atthe ground station during rainy conditions to maintain the intensity ofthe signal received by the satellite at a constant level. Thus, in clearconditions power output of the transmitter is reduced but in rainyconditions the uplink power control increases transmitter power so thatthe uplink signals penetrate the precipitation and a signal arrives atthe satellite which is generally equal to that which is provided inclear conditions.

For many duplex links designed in this way the uplink power control at aparticular site maintains uplink availability long after the downlink(that is the signal transmitted from the satellite to the groundequipment) into the same site has failed. This effect is particularlytrue for connections which are downlink power limited, even to theextent that the uplink power control serves no purpose.

The basic problem is that calculating a duplex link as two simplex linksin opposite directions ignores a fundamental principle of probabilityand results in the designs being optimistic in their availability claim.For example, assume a "duplex" circuit between two earth stations, X andY, has four component links:

Link A--Uplink from X to Satellite.

Link B--Downlink from Satellite to Y.

Link C--Uplink from Y to Satellite.

Link D--Downlink from Satellite to X.

If any one of the component links becomes unavailable the entire duplexcircuit is unavailable.

The conventional method of designing such an end to end "duplex" link orcircuit is to decide the required availability, eg 99.5%. Given thistarget Link A could be engineered to give 99.8% availability and Link Bto give 99.7%. Conventional calculations then give an end to end linkavailability of 99.5%. This is correct for the simplex path between Xand Y. To complete the conventional "duplex" link the procedure isrepeated between Y and X. For example Link C could be engineered for99.9% availability and Link D for 99.6%. Again conventional calculationsgive the end to end availability of 99.5%.

Unfortunately the conventional method ignores the fact that componentLinks A and D have dependent availability, and the same is true of LinksB and C. The basis of this dependency is that rain at Station X affectsLinks A and D (likewise at Y it affects Links B and C). In C Band, beingfar less vulnerable to rain, this has insignificant effects. However inthe higher bands, X Band and above, the effects or rain attenuation aresignificant. Reanalysing the above example as a true duplex circuitreveals that the actual end to end availability is 99.3%.

The object of this invention is to provide a method for determiningparameters for ground equipment (in particular, but not exclusively, fortransmitter power requirement and antenna size), which can minimisecapital and operational cost of an earth station.

Surprisingly, we have found by use of the method according to thisinvention that uplink power control facility can, in most cases, becompletely done away with and in other cases can be reduced to a minimumwhile still obtaining the required availability of the satellite linksystem.

SUMMARY OF THE INVENTION

The invention may be said to reside in a method of determiningparameters for the design of ground equipment for a satellite linksystem utilising duplex links comprising:

selecting a required availability of the satellite system;

choosing ground sites for ground equipment;

determining site degradation parameters for the ground sites;

determining an operating range where no uplink power control isrequired; and

determining a point of minimum transponder power;

such that parameters of the ground equipment required to transmit theduplex signal can be selected for the design of the ground equipment.

Thus, according to the invention it is possible to minimise the designof ground equipment so that minimum transponder power and minimumantenna size, for example, can be utilised while still obtaining therequired availability of the system. Thus, the cost of the ground siteinstallations is reduced as is the cost of operating those sites.

Preferably the method also includes the steps of checking that theminimum transponder power falls within the non-uplink power controlregion and if so selecting parameters for the design of the groundequipment which do not require uplink power control facility.

In other embodiments if the minimum transponder power requirement doesfall within the uplink power control region a minimum amount of uplinkpower control can be utilised to provide the required availability or anincrease in transponder power can be utilised.

In one embodiment of the invention where the point of minimumtransponder power does not fall in a range where no uplink power controlis required, the method includes a technique of iteration whereby newearth station parameters are selected until a point of minimumtransponder power for those parameters is determined which falls withinthe range where no uplink power control is required.

Preferably the step of determining the range where no uplink powercontrol is required comprises the steps of:

determining a first uplink rain attenuation function relating rainattenuation to the percentage of time for which this rain attenuation isexceeded at a first ground site;

determining a second uplink rain attenuation function relating rainattenuation to the percentage of time for which this rain attenuation isexceeded at a second ground site;

determining a first downlink rain degradation function relating raindegradation to the percentage of time for which this rain degradation isexceeded at the first ground site;

determining a second downlink rain degradation function relating raindegradation to the percentage of time for which this rain degradation isexceeded at the second site;

calculating the first and second uplink rain attenuation functions andcalculating the first and second downlink rain degradation functionsagainst outage time for the first and second sites with the seconduplink attenuation function and second downlink degradation functionhaving their Y axes drawn at a predetermined outage time of the firstuplink attenuation function and first downlink degradation function andwith the X scale inverted so that the predetermined outage time of thesecond uplink attenuation function and second downlink degradationfunction occur at zero availability for the first uplink attenuationfunction and first downlink degradation function;

and determining where the first uplink rain attenuation functionintersects the second downlink rain degradation function to define afirst point and determining where the second uplink rain attenuationfunction intersects the first downlink rain degradation function todefine a second point;

uplink power control being required at the first site if the outage timeat the first site is less than that at the first point and uplink powercontrol is not required at the second site if the outage time allocatedat the first site is less than that of the second point;

if the first point occurs at an outage time at the first site which isless than the outage time at the first site for the second point thenuplink power control is not needed at either site for outage times atthe first site between the two points; and

if the first point occurs at an outage time at the first site which isgreater than the outage time at the first site for the second point thenuplink power control is needed at both sites for outage times at thefirst site between the two points.

Preferably the system operates in any band subject to rain attenuation.

The step of determining the functions should be understood to meaneither making a physical graph or merely solving the functionsmathematically by computer or otherwise to obtain the first and secondpoints.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiment of the invention will be described in moredetail, by way of example, with reference to the accompanying drawingsin which:

FIG. 1 is a graph showing attenuation/degradation probability curves;

FIG. 2 is a graph showing joint probability curves.

DETAILED DESCRIPTION OF THE INVENTION

In the preferred embodiment of the invention the following variablefunctions are used and defined hereinunder.

f_(ru1) (x₁)=Uplink Rain Attenuation Function relating Rain Degradationto the percentage of time for which this Rain Attenuation is exceeded atsite A in Decibels.

f_(ru2) (x₂)=Uplink Rain Attenuation Function relating Rain Degradationto the percentage of time for which this Rain Attenuation is exceeded atsite B in Decibels.

f_(rd1) (x₁)=Downlink Rain Degradation Function relating RainDegradation to the percentage of time for which this Rain Degradation isexceeded at site A in Decibels.

f_(rd2) (x₂)=Downlink Rain Degradation Function relating RainDegradation to the percentage of time for which this Rain Degradation isexceeded at site B in Decibels.

x1=Percentage of downtime at site A (rain degradation limit exceeded).

x2=Percentage of downtime at site B (rain degradation limit exceeded).

TPD(x)=Relative Transponder Power as a function of rain degradation timefor carriers dimensioned for downlink requirements for a duplex circuitin dBr.

DT=Total percentage of downtime for which a duplex link is designed.

SA1=Site advantage of site A (the total effect of G/T difference andsatellite EIRP difference for that site). If the site has a totalreceive advantage then this figure is positive, if not the figure isnegative.

If x1 represents the percentage of time for which a duplex circuit isdegraded at site A, x2 represents the percentage of time for which acircuit is degraded at site B and y represents the percentage of timefor which either circuit (and therefore a duplex link) is degraded thenthe following relationship applies:

    1-y=(1-x1)(1-x2)

This can be expanded as follows;

    1-y=1-x1-x2+x1x2

Which reduces to;

    y=x1+x2-x1x2

If x1 and x2 are very small then the term x1x2 approaches zero and;

    y=x1+x2

is a good approximation; Therefore it can be stated that;

    DT=x1+x2

And;

    x2=DT-x1.

For example if a required availability of 99.5% is selected thendowntime equals 0.5%. Uplink frequencies of approximately 14.25 GHz areselected. Site A and site B are known by their latitude and longitudeand site degradation parameters (that is rainfall data) for those sitesis obtained from data banks, CCIR Rain Model or other more improveddata. Furthermore, an initial transponder power requirement is assumedand, for example, both site A and site B transponders might require tenwatt transmitters. The downlink rain degradation functions can beexpressed as follows:

    f.sub.rd1 (x1)=f.sub.rd1 (DT-x2)                           1

    f.sub.rd2 (x2)=f.sub.rd2 (DT-x1)                           2

For carriers dimensioned for a fully downlink rain degradation limitedsituation the relative transponder power is given by: ##EQU1##

The site A downtime x1 for which the transponder power is minimum isfound by solving:

    TPD'(x1)=0;                                                (4)

As both degradation functions are usually determined empirically it isextremely difficult to solve equation 4 above. However, a solution canbe found using conventional numerical computing methods. This method canbe applied graphically. The functions f_(rd1) (x1) and f_(rd2) (DT-x1)are plotted on the same axes together with f_(ru1) (x1), f_(ru2) (DT-x1)and TPD(x1). From this it is possible to determine the two carrier sizesnecessary at any site downlink rain degradation combination which givesthe required duplex link availability. It is also possible to determinewhether or not the level of a particular carrier is sufficient for theuplink attenuation expected at the transmitting site. If the uplink isinsufficient, uplink power control will be needed at the uplinking site.From the relative transponder power curve it is possible to determinethe site outage times at which the transponder power is minimum, and theexcess transponder power needed for any other operating point.

To determine the downlink degradation function it is necessary to sumthe effects of downlink attenuation and the noise temperaturedegradation which results from the elevated temperature of theattenuating rain. The noise temperature degradation effect is dependenton the receive system noise temperature as well as the levels of uplinkand transponder intermodulation noise.

With reference to FIG. 1 which shows typical downlink and uplinkattenuation functions as well as the downlink degradation function. Thisis for a typical Ku band downlink limited link where the rain derivedreceived noise temperature results in the downlink degradation exceedingthe uplink attenuation at the same site. The downlink attenuation islower because the attenuation at 12 GHz is lower than the attenuation at14 GHz.

FIG. 2 shows two sets of downlink degradation and uplink attenuationcurves, the first of which is as for FIG. 1. The second curve has its Yaxis drawn at the 0.5% outage time of the original graph and the X scaleis inverted so that for the two right hand curves, 0.5% availabilityoccurs at 0 availability for the first curve. An availability of 99.5%is used in this example but the method is applicable to any availabilitylevel.

With reference to FIG. 2, two important points can be observed. Firstly,the point at which the transponder power is minimised (point A) andsecondly, the point at which the two downlink degradation lines cross(point D). The latter point is the point at which the rain fade marginfor both carriers would need to be equal. Also, another importantfeature of the transponder power curve is that it is extremely flat withvery little increase in transponder power on either side of the minimumpoint. This power curve example is for two sites having equal siteperformance, if one site has an advantage this power curve is skewed toone side. It is interesting to note from this example that if the pointof equal downlink rain degradation is chosen as the operating point, thestation having the more favourable weather conditions would be operatingwith a higher path availability (in this case 99.865%) whilst thestation experiencing bad weather conditions would have only 99.635% linkavailability. At this operating point, the transponder utilisation isonly marginally more than for the minimum point of the transponderpower.

The graph of FIG. 2 enables a range to be determined where no uplinkpower control is required and that range is between the points B and Cin FIG. 2. In the examples shown the point of minimum transponder powerfalls within that range and therefore selected parameters relating tothe design of the ground equipment such as transponder power requirementand antenna size can be selected to be a minimum in order to provide theavailability which is required and therefore the cost of installing theground site and also operation of the ground site can be minimised.

It should be noted that in FIG. 2 the shaded area represents regionswhere uplink power control would be required in order to provide thedesired availability of the system. Conventional methods for determiningthe parameters of designs result in uplink power control facilitiesbeing required in order to provide the required availability of thesystem. Thus, the preferred embodiment of the invention provides amethod whereby in many cases it will be possible to design installationswhich do not require uplink power control and which therefore are ofreduced installation cost and also reduced operational cost.

The effect of moving the transponder operating point away from point Dshown in FIG. 2 will now be described. Firstly if an operating point ischosen for line Z, station A must transmit a very large signalillustrated by point E to cover the rain attenuation expected at site B.This signal being uplinked from site A would only experience attenuationas shown at point F, therefore this carrier being uplinked from stationA to a station at site B would not require uplink power control tooperate. However, the return circuit (the signal transmitted from thestation at site B to the station at site A) would need to transmit acarrier at the level illustrated at point G. This signal being uplinkedfrom site B would experience more attenuation than the degradation ofits downlink. It would therefore be necessary for uplink power controlto be provided at site B to ensure that the required performance isachieved.

The amount of the uplink power control needed to achieve this is theamount illustrated by the difference in the attenuation at points G andH (in this case 8 dB). The amount of transponder power needed to supportthis operating point is actually 3.0 dB higher than the minimum possible(Point A).

Thus should the point of minimum transponder power fall in the regionwhere uplink power control is required design parameters can be selectedto result in the minimum amount of uplink power control beingincorporated in the system or an increase in the transponder power couldbe selected so as to bring the point A back into the region where nouplink power control is required.

The next important point to observe is the point which is intersected byline X. At this point the downlink required in to site A suffers thesame attenuation as its associated uplink being transmitted from site B.This circuit therefore does not need uplink power control as both theuplink and associated downlink would fade at the same time. It is alsoimportant to note that the signal transmitted from site A to site Bwould experience more downlink attenuation than uplink attenuation, andtherefore this carrier would also not need uplink power control tooperate at this point.

The same conditions as illustrated for lines X and Z occur to the leftof the equal downlink fade degradation point as on the right. It istherefore important to note that for lines between points B and C, nouplink power control is required to achieve operation. It is alsoimportant to note that the minimum transponder power usually fallsbetween points B and C. This means that there is no advantage in the useof uplink power control as it does not reduce the transponder powerrequirement. When an uplink power control advantage is indicated, it isusually very small, less than 0.5 DB.

For simplex circuits, much larger uplink power control advantages couldbe obtained because the transmitting site has no receive responsibility.For a duplex circuit, the advantage of the uplink power control is notrealised because the duplex availability at the transmit site is limitedby the availability of the downlink at the same site.

At a point, such as that illustrated by Point A, the complementary sitedownlink availabilities are Site B--99.655% (Point J) and SiteA--99.845% (Point L). The Site A uplink power margin required to offsetthe fade is 12.5 dB with the result that Site A's uplink availability is99.915% (Point M). Similarly the Site B uplink power margin required toovercome the fade margin is 11.5 dB which gives Site B an uplinkavailability of 99.745% (Point K).

The preferred embodiment in terms of a mathematical description of themethod can be described using the same variable functions as definedherein.

The mathematical solution is as follows:

1. Solve TPD'(x1)=0 to obtain x_(1m) which is minimum Transponder powerpoint.

2. Solve f_(ru1) (x1)=f_(rd2) (x2) to obtain x_(1L) which is lower limitof non uplink power control.

3. Solve f_(rd1) (x1)=f_(ru2) (x2) to obtain x_(1u) which is upper limitof non uplink power control.

4. If x_(1L) ≦x_(1m) ≦x_(1u) is true, then configure links using x_(1m)as the operating point.

5. If x_(1L) >x_(1m), then configure links using x_(1L) as the operatingpoint.

6. If x_(1u) <x_(1m), then configure links using x_(1u) as the operatingpoint.

7. If condition in 5 or 6 exist, then antenna gains can be increased ateither end. The cost of this increased gain would then be compared tothe cost of using the additional transponder power that 5 or 6 requireto find the most cost effective solution.

8. If x_(1u) ≦x_(1L) then configure links using x_(1m) as the operatingpoint. Use the difference between f_(rd1) (x1) and f_(ru2) (x2) todetermine the amount of uplink power control required at site 1 andlikewise the difference between f_(ru1) (x1) and f_(rd2) (x2) todetermine the amount of uplink power control at site 2. If the amount ofrequired uplink power control required is small, it may be cheaper touse more transponder power or improve the antenna gains at either orboth of the sites, thereby producing within the method a range ofoperating points where no uplink power control is required, as above.

Advantages of the preferred embodiment of the invention are as follows:

(a) the best possible use is made of the statistical rainfall data todimension carriers for duplex links;

(b) the carrier sizes allocated are more uniform across the transponder,meaning that smaller carriers suffer less gain compression, and are lesssubject to adjacent carrier interference;

(c) no carriers with very small margins are allocated. Such smallcarriers are more prone to losses in continuity which may result fromsmall degradations in earth station performance such as could be causedby obstructions falling on the antenna, insects building their nest inthe antenna feed or similar events;

(d) the clear sky operating bit error rates are optimized. If one of thecarriers is much smaller than the other, the bit error rate on thatcarrier would be very high while the other would achieve a much lowerbit error rate. For this design the bit error rate is optimised for bothhalves of the link;

(e) the availabilities of the two circuits in a duplex link have maximumoverlap; and

(f) the identification of the true value of Uplink Power Control enablesthe design to minimise the capital cost of ground equipment by avoidingthe use of UPC when no significant advantage arises from its use; and

(g) transponder power utilisation is minimised.

The most important result of this method of rain fade margin allocationis the observation that uplink power control for duplex circuits is ofsuch little use as to represent no economic or other advantage for earthstations which are downlink thermal noise limited having similar siteadvantages. The method can be used to optimise circuits with;intermodulation or uplink noise limitations; or large differences insite performance, but in these cases, it may be necessary to employ UPCin order to minimise transponder utilisation.

Since modifications within the spirit and scope of the invention mayreadily be effected by persons skilled within the art, it is to beunderstood that this invention is not limited to the particularembodiment described by way of example hereinabove.

We claim:
 1. A method of determining parameters for the design of groundequipment for a satellite link system utilising duplex linkscomprising:selecting a required availability of the satellite system;choosing at least first and second ground sites for ground equipment;determining site degradation parameters for the ground sites;determining an operating range where no uplink power control isrequired; and determining a point of minimum transponder power; suchthat parameters of the ground equipment required to transmit the duplexsignal can be selected for the design of the ground equipment.
 2. Themethod of claim 1, including the steps of checking that the minimumtransponder power falls within the non-uplink power control region andif so selecting parameters for the design of the ground equipment whichdo not require uplink power control facility.
 3. The method of claim 1,wherein if the minimum transponder power requirement does fall withinthe uplink power control region a minimum amount of uplink power controlcan be utilized to provide the required availability or an increase intransponder power can be utilized.
 4. The method of claim 1, wherein ifthe point of minimum transponder power does not fall in a range where nouplink power control is required, the method includes a technique ofiteration whereby new site degradation parameters are selected until apoint of minimum transponder power for those parameters is determinedwhich falls within the range where no uplink power control is required.5. The method of claim 1, wherein the step of determining the rangewhere no uplink power control is required comprises the stepsof:determining a first uplink rain attenuation function relating rainattenuation to the percentage of time for which this rain attenuation isexceeded at a first ground site; determining a second uplink rainattenuation function relating rain attenuation to the percentage of timefor which this rain attenuation is exceeded at a second ground site;determining a first downlink rain degradation function relating raindegradation to the percentage of time for which this rain degradation isexceeded at the first ground site; determining a second downlink raindegradation function relating rain degradation to the percentage of timefor which this rain degradation is exceeded at the second site;calculating the first and second uplink rain attenuation functions andcalculating the first and second downlink rain degradation functionsagainst outage time for the first and second sites with the seconduplink attenuation function and second downlink degradation functionhaving their Y axes drawn at a predetermined outage time of the firstuplink attenuation function and first downlink degradation function andwith the X scale inverted so that the predetermined outage time of thesecond uplink attenuation function and second downlink degradationfunction occur at zero availability for the first uplink attenuationfunction and first downlink degradation function; and determining wherethe first uplink rain attenuation function intersects the seconddownlink rain degradation function to define a first point anddetermining where the second uplink rain attenuation function intersectsthe first downlink rain degradation function to define a second point;uplink power control being required at the first site if the outage timeat the first site is less than that at the first point and uplink powercontrol is not required at the second site if the outage time allocatedat the first site is less than that of the second point; if the firstpoint occurs at an outage time at the first site which is less than theoutage time at the first site for the second point then uplink powercontrol is not needed at either site for outage times at the first sitebetween the two points; and if the first point occurs at an outage timeat the first site which is greater than the outage time at the firstsite for the second point then uplink power control is needed at bothsites for outage times at the first site between the two points.
 6. Themethod of claim 1, wherein the system operates in any band subject torain attenuation.
 7. The method of claim 1, wherein the range where nouplink power control is required is calculated as follows:a. solveTPD'(x1)=0 to obtain x_(1m) which is minimum Transponder power point, b.solve f_(ru1) (x1)=f_(rd2) (x2) to obtain x_(1L) which is lower limit ofnon uplink power control, c. solve f_(rd1) (x1)=f_(ru2) (x2) to obtainx_(1u) which is upper limit of non uplink power control, d. if x_(1L)≦x_(1m) ≦x_(1u) is true, then configure links using x_(1m) as theoperating point, e. if x_(1L) >x_(1m), then configured links usingx_(1L) as the operating point, f. if x_(1u) <x_(1m), then configurelinks using x_(1u) as the operating point, g. if condition in e or fexist, then antenna gains can be increased at either end, h. if x_(1u)≦x_(1L) then configure links using x_(1m) as the operating point and usethe difference between f_(rd1) (x1) and f_(ru2) (x2) to determine theamount of uplink power control required at the first site and likewisethe difference between f_(ru1) (x1) and f_(ru2) (x2) to determine theamount of uplink power control at the second site.
 8. A method oftransmitting signals in a satellite link system utilizing duplex linkscomprising:selecting a required availability of the satellite system;determining site degradation parameters for first and second groundsites; determining an operating range where no uplink power control isrequired; determining a point of minimum transponder power; andtransmitting the signals from the first ground site to the second groundsite via the satellite system.
 9. The method of claim 8, furthercomprising the steps of checking whether the minimum transponder powerfalls within the non-uplink power control region and, if so,transmitting the signals without uplink power control.
 10. The method ofclaim 8, wherein if the minimum transponder power requirement does fallwithin the uplink power control region a minimum amount of uplink powercontrol can be utilized to provide the required availability or anincrease in transponder power can be utilized.
 11. The method of claim8, wherein if the point of minimum transponder power does not fall in arange where no uplink power control is required, the method includes atechnique of iteration whereby new site degradation parameters areselected until a point of minimum transponder power for those parametersis determined which falls within the range where no uplink power controlis required.
 12. The method of claim 8, wherein the step of determininga range where no uplink power control is required comprises the stepsof:determining a first uplink rain attenuation function relating raindegradation to the percentage of time for which this rain degradation isexceeded at a first ground site; determining a second uplink rainattenuation function relating rain attenuation to the percentage of timefor which this rain attenuation is exceeded at a second ground site;determining a first downlink rain degradation function relating rainattenuation to the percentage of time for which this rain attenuation isexceeded at the first ground site; determining a second downlink raindegradation function relating rain degradation to the percentage of timefor which this rain degradation is exceeded at the second site;calculating the first and second uplink rain attenuation functions andcalculating the first and second downlink rain degradation functionsagainst outage time for the first and second sites with the seconduplink attenuation function and the second downlink degradation functionhaving their Y axes drawn at a predetermined outage time of the firstuplink attenuation function and first downlink degradation function andwith the X scale inverted so that the predetermined outage time of thesecond uplink attenuation function and second downlink degradationfunction occur at zero availability for the first uplink attenuationfunction and first downlink degradation function; and determining wherethe first uplink rain attenuation function intersects the seconddownlink rain degradation function to define a first point anddetermining where the second uplink rain attenuation function intersectsthe first downlink rain degradation function to define a second point;uplink power control being required at the first site if the outage timeat the first site is less than that at the first point and uplink powercontrol is not required at the second site if the outage time allocatedat the first site is less than that of the second point; if the firstpoint occurs at an outage time at the first site which is less than theoutage time at the first site for the second point then uplink powercontrol is not needed at either site for outage times at the first sitebetween the two points; and if the first point occurs at an outage timeat the first site which is greater than the outage time at the firstsite for the second point then uplink power control is needed at bothsites for outage times at the first site between the two points.
 13. Themethod of claim 8, wherein the system operates in any band subject torain attenuation.
 14. The method of claim 8, wherein the range where nouplink power control is required is calculated by performing thefollowing steps:a. solving TPD' (x1)=0 to obtain x_(1m) which is theminimum transponder power point; b. solving f_(ru1) (x1)=f_(rd2) (x2) toobtain x_(1L) which is a lower limit of non-uplink power control; c.solving f_(rd1) (x1)=f_(ru2) (x2) to obtain x_(1u) which is an upperlimit of non-uplink power control; d. if x_(1L) ≦x_(1m) ≦x_(1n) is true,then configuring links using x_(1m) as the operating point; e. if x_(1L)>x_(1m), is true, then configuring links using x_(1L) as the operatingpoint; f. if x_(1u) <x_(1m), is true, then configuring links usingx_(1u) as the operating point; g. if conditions determined in e or fexist, then increasing antenna gains at either end; h. if x_(1u) ≦x_(1L)is true, then configuring links using x_(1m) as the operating point andusing the difference between f_(rd1) (x1) and f_(ru3) (x2) to determinethe amount of uplink power control required at the first site andlikewise the difference between f_(ru1) (x1) and f_(rd2) (x2) todetermine the amount of uplink power control at the second site.